To meet future EPA emissions standards, large bore medium speed diesel engines need greater flexibility and reliability on the fuel injection equipment with regard to fuel metering, injection timing, injection pressure, rate of injection (rate shaping) and multiple (pre-, post-, or split) injections independent of engine speed. A production unit pump system (UPS) offers the advantage of design simplicity with flexibility on electronically-controlled injection timing.
However, the rate of injection and injection pressure are solely dependent upon cam profile and engine speed, and are optimized for full load operating conditions. It is impossible to provide split injections and/or injection pressure control and pressure level. At engine idle and lower speeds, the UPS cannot generate adequate high injection pressures that are necessary to achieve complete combustion.
In order to overcome these shortcomings, advancements to the UPS such as current controlled rate shaping (CCRS) and advanced unit pump system (AUPS) are being developed by the fuel injection equipment manufacturers. For example, see International Council on Combustion Engines, 2001 Congress, Hamburg, Germany, pages 511 through 517. Also, several new injection systems, such as common rail system (CRS) and amplifier piston common rail system (APCRS), are currently being developed. The CCRS concept offers the advantage of excellent retrofit capability with incremental cost, and it can provide initial injection rate shaping (boot injection) but limited by the cam profile, engine speed, and needle valve opening pressure. The AUPS can provide controlled injection pressure that is independent of engine speed. However, it cannot provide split injections that are essential to reduce certain exhaust emissions, engine noise, and improve fuel efficiency.
The CRS and APCRS systems (both being non-UPS) offer flexibility to control the injection timing and injection pressure independent of cam profile and/or engine speed. However, the high pressure CRS allows only controlling simple and multiple injections. Higher pressure peaks at the end of injection event and pressure pulsations at higher injection quantities limit its application to medium speed, large bore diesel engines. The APCRS, using either hydraulically- or mechanically-controlled pressure amplifier concepts, has the potential to permit pre- and post-injections, and variations in injection rate shaping. However, the boot pressure ratio is not variable because of a geometrically-fixed amplification ratio.
A more critical hardware constraint is the layout of the low-pressure system avoiding pressure pulsations. In comparison, the production UPS and the delineated alternative fuel systems cannot offer the desired flexible injection (cam and speed independent injection pressure, rate shaping, and multiple injections) while maintaining the reliability and cost effective retrofit capability.
Accordingly, what remains needed in the art is a fuel injection system which is a hybrid, the system allowing electronic control over the injection processes, wherein selective rate shaping and multiplicity of injections is made possible.